This invention relates to a lead structure for packaging a semiconductor chip comprising a large number of electroconductive leads, applicable to a tape carrier of TAB (Tape Automated Bonding) method, for example.
TAB method is known as one of the packaging techniques in manufacturing an integrated circuit (IC). For example, it is disclosed in U.S. Pat. No. 3,689,991.
In TAB method, a tape carrier 101 is used for supporting a semiconductor chip 110 as shown in FIG. 6. The tape carrier 101 comprises a film tape 102 and a large number of electroconductive lead patterns 103 formed on the film tape 102. The tape carrier 101 is provided with sprocket holes 106. The film tape 102 has a support ring 107 which defines a device aperture 104 for the semiconductor chip 110 at its center and an outer lead aperture 105 between the support ring 107 and the major part of the film tape 102 surrounding the support ring 107. Each lead pattern 103 is generally classified into inner lead portion 108 and outer lead portion 109. The inner lead portions 108 of the lead patterns 103 are projected into the device aperture 104 of the support ring 107 while the outer lead portions 109 are extended over the outer lead aperture 105. After the semiconductor chip 110 is bonded to the inner lead portions 108 in the device aperture 104, the outer lead portions 109 are cut at their outer ends in the outer lead aperture 105. In this manner, the semiconductor chip 110 and the support ring 107 connected to each other by the inner lead portions 108 are separated together with the outer lead portions 109 from the tape carrier 101. Then, the outer lead portions 109 which project out from the support ring 107 are bent in a so-called gull wing shape and bonded to wiring patterns of a printed board or the like.
In such TAB method, the tape carrier 101 can be fed and positioned with high accuracy utilizing the sprocket holes 106. A large number of fine lead patterns can be formed. Batch bonding can be carried out on the inner lead portions 108 and the outer lead portions 109, respectively. Therefore, a complete automation of the packaging process of semiconductor chips can be attained.
For sealing the semiconductor chip 110 with plastics in such TAB method, there has heretofore been adopted a potting method wherein the semiconductor chip 110 is covered with a plastic coating 136 as indicated by alternate long and two short dashes line in FIGS. 6 and 7. In that potting method, however, a multi-layer structure is formed wherein the support ring 107 and the lead patterns 103 thereon are sandwiched by the upper and lower parts of the outer peripheral portion of the plastic coating 136. Thus, the upper and lower parts of the plastic coating 136 do not contact each other in that portion. As a result, problems such as easy entry of moisture or contaminants and easy separation of the plastic coating 136 have been encountered.
For that reason, in recent years, TAB method has employed a transfer molding technique, in which the semiconductor chip 110 is sealed or encapsulated by a plastic encapsulation 137 formed by a mold 120 as shown in FIG. 7. The plastic encapsulation 137 includes the base portions 109a of the outer lead portions 109. In this manner, the outer peripheral portion of the plastic encapsulation 137 becomes integral so it is possible to positively prevent entry of moisture and contaminants and also prevent separation of the plastic encapsulation 137.
However, where the outer lead portions 109 are formed at a narrow pitch, when the plastic encapsulation 137 is formed by the mold 120, a fine gap 138 is formed between adjacent outer lead portions 109 sandwiched by pressing surface 123 of an upper mold half 121 and a pressing surface 124 of a lower mold half 122 as shown in FIG. 8. When the the plastic encapsulation 137 is formed, since the pressure of molten plastics injected into concave portions 125 and 126 forming a cavity of the mold 120 is very high, the molten plastics flows out of the cavity into the fine gaps 138, resulting in oozes 139 between the outer lead portions 109 as shown in FIGS. 6 and 8. Such ooze 139 solidify to form flashes. The flashes 139 between the outer lead portions 109 are easily carbonized by heat during the subsequent outer lead bonding step and short-circuit the outer lead portions 109. Where the flashes 139 between the outer lead portions 109 are blackened, is unsightly and it's value is deteriorated. Further, the flashes 139 cause abrasion of the mold used for bending the outer lead portions 109 into the gull wing shape because the mold bites the flashes 139 together with the outer lead portions 109.
Although such outflow of molten plastics is apt to occur when the viscosity of the molten plastics is low or the injecting pressure thereof is high in transfer molding or injection molding, it is suspected that the reason why the molten plastics reaches near the front ends of the outer lead portions 109 is the capillarity in the fine gaps 138. Therefore, it is difficult to prevent the oozes 139 by simply adjusting the viscosity of molten plastics or the injecting pressure thereof.